(1) Field of the Invention
The present invention relates to a mercury recovery method and a mercury recovery apparatus, and particularly to a technique for separating and collecting mercury contained in phosphors used in fluorescent lamps.
(2) Related Art
Mercury, which is a toxic material, is used in lighting devices such as fluorescent lamps. When fluorescent lamps whose life is expired or fluorescent lamps that are found defective during manufacturing processes (hereafter simply referred to as “waste fluorescent lamps”) are disposed, mercury is usually to be recovered from the waste fluorescent lamps. The recovered mercury is processed to remove impurities etc., and then is recycled.
As a method for recovering mercury from phosphor powder used in waste fluorescent lamps, the wet-method is conventionally known, one example of which is disclosed in Japanese laid-open patent application No. H10-12149. According to the disclosed method, mercury-containing phosphor powder detached from a waste fluorescent lamp is immersed in an aqueous solution in which active sulfur is dissolved, to form a mercuric sulfide compound that is refractory, so that mercury can be recovered from the phosphor powder.
Also, as another method for recovering mercury from phosphor powder, the dry-method is conventionally known, one example of which is disclosed in Japanese published examined patent application No. S53-1594. This method takes advantage of a characteristic of mercury as being a low-boiling metallic element. According to the disclosed method, mercury-containing phosphor powder is subjected to a heating process to vaporize mercury contained therein, and the vaporized mercury is then cooled and condensed, so that mercury can be recovered from the phosphor powder.
Here, rare-earth phosphors, which are expensive, are often used in fluorescent lamps for general lighting. When waste fluorescent lamps are disposed, therefore, it is desirable to recycle not only mercury, but also phosphor powder from which mercury is separated (hereafter simply referred to as “treated phosphor powder”).
However, with the above-described conventional wet-method, a portion of the phosphor powder is dissolved into the aqueous solution containing active sulfur, with the crystal structure of phosphors being destroyed or changed. This causes characteristics of the phosphor powder including luminance to deteriorate, making recycling of the treated phosphor powder impossible.
Further, unlike the dry-method mercury recovery, the wet-method mercury recovery generally requires special equipment for liquid-waste treatment, including treatment of used aqueous solutions etc. Therefore, the wet-method mercury recovery tends to suffer from high-cost, compared with the dry-method mercury recovery.
With the conventional dry-method mercury recovery, substantially all mercury, when being contained in the form of metallic mercury or mercurous oxide, can be separated and collected from phosphor powder. However, the problem is that phosphor powder of life-expired fluorescent lamps, in particular, contain mercury in the form of an amalgam that is formed by reacting with an emitter (emissive material) or a phosphor element. It is difficult to decompose an amalgam by heat, and accordingly it is difficult to separate and collect substantially all mercury from such phosphor powder. To solve this problem, increasing a heating temperature in the heating process may be considered. However, the temperature being too high in the heating process causes thermal degradation of phosphor powder, which results in characteristics of the phosphor powder including luminance deteriorating, making recycling of the treated phosphor powder impossible.